The present invention relates generally to methods for removing iron oxide scale from processed sheet metal and inhibiting further oxidation in the processed sheet metal. More particularly, the present invention relates to methods for removing iron oxide scale from the surfaces of processed sheet metal using a mechanical surface conditioning apparatus in a manner to inhibit further oxidation on the conditioned surfaces and to reduce surface roughness.
Processed sheet metal has a wide variety of applications. For example, aircraft, automobiles, file cabinets and household appliances, to name only a few, contain sheet metal bodies or shells. The sheet metal is typically purchased directly from steel mills and/or steel service centers, but may be passed through intermediate processors (sometimes referred to as xe2x80x9ctollxe2x80x9d processors) before it is received by an original equipment manufacturer. Sheet metal is typically formed by hot rolling process and, if the gauge is thin enough, it is coiled for convenient transport and storage. During the hot rolling process, carbon steel typically reaches finishing temperatures well in excess of 1500xc2x0 F. (815xc2x0 C.). Once the hot rolling process is completed, the hot rolled steel is reduced to ambient temperature, typically by quenching in water, oil or polymer, as is well known in the art. As a result of reactions with oxygen in the air and moisture, an iron oxide layer (or xe2x80x9cscalexe2x80x9d) is formed on the surface of hot rolled carbon steel while the steel is cooled. The rate at which the product is cooled, and the total temperature drop, will affect the amount and composition of scale that forms on the surface during the cooling process.
Iron has a complex oxide structure with FeO (xe2x80x9cwustitexe2x80x9d) mechanically bonded to the base metal substrate, followed by a layer of Fe3O4 (xe2x80x9cmagnetitexe2x80x9d) chemically bonded to the wustite, and then a layer of Fe2O3 (xe2x80x9chematitexe2x80x9d) chemically bonded to the magnetite and which is exposed to the air. Oxidation tends to progress more rapidly at higher temperatures, such as those reached in a typical hot rolling process, resulting in the formation of wustite. The relative thickness of each of the distinct wustite, magnetite and hematite layers is related to the availability of free oxygen and iron as the hot rolled substrate cools. When cooled from finishing temperatures above 1058xc2x0 F. (570xc2x0 C.), the oxide layer will typically comprise at least 50% wustite, and will also comprise magnetite and hematite in layers, formed in that order from the substrate. Though a number of factors (e.g., quenching rate, base steel chemistry, available free oxygen, etc.) affect the relative thicknesses of wustite, magnetite and hematite, as well as the overall thickness of the oxide layer, research has shown that the overall thickness of the oxide layer (inclusive of all three of these layers) in hot rolled carbon steel will typically be about 0.5% of the total thickness of the steel sheet. Thus, for example, in xe2x85x9cxe2x80x3 hot rolled carbon steel, the overall thickness of the oxide layer will be about 0.002xe2x80x3.
Various methods exist for flattening sheet metal and for conditioning the surfaces thereof. Flatness of sheet metal is important because virtually all stamping and blanking operations require a flat sheet. Good surface conditions are also important, especially in applications where the top and/or bottom surfaces of the metal sheet will be painted or otherwise coated. For processed sheet metal that is to be painted or galvanized, current industry practice is to remove all evidence of oxide from the surface to be painted or galvanized. With respect to painted surfaces, removing all evidence of oxide before painting ensures optimum adhesion, flexibility, and corrosion resistance of the intended paint coating layer. With respect to galvanizing, removing all evidence of oxide before coating allows a sufficient chemical bond of zinc to base metal.
The most common method of removing all oxide from the surface of hot rolled sheet metal before coating is a process known as xe2x80x9cpickle and oil.xe2x80x9d In this process, the steel (already cooled to ambient temperature) is uncoiled and pulled through a bath of hydrochloric acid (typically about 30% hydrochloric acid and 70% water) to chemically remove the scale. Then, after the scale has been removed, the steel is washed, dried, and immediately xe2x80x9coiledxe2x80x9d to protect it from rust damage. The oil provides an air barrier to shield the bare metal from exposure to air and moisture. It is critical that the metal be oiled immediately after the pickling process, as the bare metal will begin to oxidize very quickly when exposed to air and moisture. The xe2x80x9cpickle and oilxe2x80x9d process is effective in removing substantially all of the oxide layer, including the tightly bonded wustite layer, and results in a surface that is suitable for most coating applications. However, the xe2x80x9cpickle and oilxe2x80x9d process has a number of disadvantages. For example, the oil applied to the metal after pickling must be removed before coating, which is time consuming. Also, hydrochloric acid is an environmentally hazardous chemical, which has special storage and disposal restrictions. In addition, the oil coating interferes with some manufacturing processes, such as welding, causes stacked sheets to stick together, and gets into machine parts during manufacturing processes. Also, while the pickling process is effective at removing substantially all of the oxide layer, resulting in a surface that is suitable for most coating applications, the pickling agent (hydrochloric acid) tends to leave a clean but slightly coarse surface.
Thus, there is a need for an improved method of surface conditioning processed sheet metal, which removes enough scale from the surface to ensure optimum conditions for accepting coatings, which results in a smooth surface that is suitable for virtually all coating applications, which includes a means for inhibiting further oxidation prior to coating, and which is less expensive and troublesome than standard pickling and oiling.
It is therefore an object of the present invention to provide an improved method of removing iron oxide scale from processed sheet metal in a manner to ensure optimum surface conditions for accepting paint, galvanizing, or other coating. A related object is to provide an improved method of removing iron oxide scale from processed sheet metal, which results in a smooth surface that is suitable for virtually all coating applications. Another object is to provide an improved method of removing iron oxide scale from processed sheet metal in a manner that will inhibit further oxidation without the need to coat with oil. Still another general object is to provide an improved method of removing iron oxide scale from processed sheet metal, which is less expensive and troublesome than standard pickling and oiling.
The present invention includes methods of removing iron oxide scale from processed sheet metal, wherein the iron oxide scale generally comprises three layers: a wustite layer, a magnetite layer, and a hematite layer. The wustite layer is bonded to a base metal substrate of the processed sheet metal. The magnetite layer is bonded to the wustite layer, and the hematite layer is bonded to the magnetite layer. In general, the methods comprise the steps of: providing a surface conditioning apparatus; and conditioning a surface of the processed sheet metal with the surface conditioning apparatus. The surface conditioning apparatus has at least one surface conditioning member. The step of conditioning the surface of the processed sheet metal includes bringing the at least one surface conditioning member into engagement with the surface of the sheet metal. The surface conditioning member is brought into engagement with the surface in a manner to remove substantially all of the hematite layer and magnetite layer from the surface. Additionally, the surface conditioning member is brought into engagement with the surface in a manner to remove some but not all of the wustite layer from the surface, so that a portion of the wustite layer remains bonded to the base metal substrate of the processed sheet metal.
In another aspect of the invention, methods of removing iron oxide scale from processed sheet metal comprise the steps of: providing a surface conditioning apparatus having at least one rotating conditioning member; and conditioning a surface of the processed sheet metal with the surface conditioning apparatus. The step of conditioning the surface of the processed sheet metal includes bringing the at least one rotating conditioning member into engagement with the surface of the sheet metal. The rotating conditioning member is brought into engagement with the surface in a manner to remove some, but less than substantially all of the iron oxide scale from the surface so that a layer of oxide scale remains bonded to a base metal substrate of the processed sheet metal. Additionally, the rotating conditioning member is brought into engagement with the surface in a manner to reduce an arithmetic mean of distances of departure of peaks and valleys on the surface, measured from a mean center line, to less than 50 micro inches.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding and appreciation of the invention may be attained by referring to the Figures and detailed description of the preferred embodiments, which follow.